The Dallas Cowboys, bought by Jerry Jones for $US150 million in 1989, are now valued at $2.1 billion ($A2.01 billion). Twenty of the league's 32 teams are valued at $US1 billion or more.

Eight of the country's top 15 most-watched TV programs were Super Bowls, and more than 100 million people around the globe are expected to tune in for the match-up between the Baltimore Ravens and San Francisco 49ers on Monday (Melbourne time).

Even after the labour meltdown and player lockout of 2011, when another league might have lost legions of fans, the NFL had a typically captivating season and grew in influence and popularity.

But fissures have formed in the once-pristine NFL edifice. More than 2000 former players are suing the league over head injuries, and what they were and weren't told about the long-term damage of concussions.

Junior Seau, among the greatest linebackers in history, committed suicide last year and was later found to have a concussion-related brain disease. Seau's family this week filed a wrongful-death lawsuit against the league. A study released last week shows signs of an ailment similar to Seau's in five living NFL alumni.

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''The culture of the athlete is still too much of a play-through-it, rather than player-safety mentality,'' NFL commissioner Roger Goodell said in November in a speech to the Harvard School of Public Health.

''Many players have publicly admitted to hiding concussions and other head injuries … This is unfortunate, but we are working with players, team doctors and coaches to change that culture.''

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The NFL is considering the drastic move of doing away with kick-offs in the name of player safety. But Goodell and team owners also have explored the possibility of expanding the regular season from 16 to 18 games, potentially increasing the likelihood of injuries. There also have been discussions about expanding the play-off field from 12 to 14 or 16 teams.

''There's an uneasy feeling around the NFL, because although the league is arguably more popular than it's ever been before, there are also these glaring areas of deep concern about player safety on the field, and the players' health off the field and after their careers are over,'' said Michael MacCambridge, author of America's Game: The Epic Story of How Pro Football Captured a Nation.

''I'm convinced that the NFL gets it, and is working very hard to make the game safer. But if you're a fan, you have to be concerned about some of the trial balloons that have been floated: an 18-game regular season is not just a bad idea for the people who play the game and watch the game, it's also totally out of step with the cultural mood of the moment. You want to believe that the owners are guided not only by revenue figures but also the greater good of the game.''

The NFL is an incredibly robust enterprise, one that generates $US9 billion a year in revenues. It probably would survive the potentially huge damages those class-action lawsuits could bring. But say the league did have to pay a staggering amount in damages. Would high schools face prohibitive insurance premiums to keep football programs going? The NFL lawsuits have a devastating ripple effect.

Already, the league has made several rules changes to protect players, and more are in the offing, angering purists who say the sport has already been tweaked and twisted too much.

''We don't want to take physical contact out of the game,'' Goodell said. ''But we must ensure that players follow rules designed to reduce the risk of injury.''

The health and safety of players is one of many challenges the league is facing. Eighteen months after their labour fight was resolved, a palpable tension remains between the NFL and the NFL Players Association. The sides have yet to agree on how to test for illicit use of human growth hormone, even though there are strong suspicions some players are using it.

The NFL and NFLPA might have achieved a breakthrough this month on HGH testing, with both sides saying they would be amenable to the type of testing done in Major League Baseball.

But they have been close to an agreement in the past before digging in and failing to put a testing plan in place. The suspicion is that both sides are reticent to begin testing because of the possibility of widespread HGH use in the league and the fallout that could bring.